Communication devices such as terminals are also known as e.g. User Equipments (UE), mobile terminals, wireless terminals and/or mobile stations. Terminals are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two terminals, between a terminal and a regular telephone and/or between a terminal and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Terminals may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The terminals in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by an access node such as a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. evolved NodeB “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
The 3GPP initiative “License Assisted Access” (LAA) intends to allow LTE equipment to also operate in the unlicensed 5 GHz radio spectrum. The unlicensed 5 GHz spectrum is used as a complement to the licensed spectrum. Accordingly, devices may connect in the licensed spectrum, primary cell or PCell, and use carrier aggregation to benefit from additional transmission capacity in the unlicensed spectrum, secondary cell or SCell. To reduce the changes that may be required for aggregating licensed and unlicensed spectrum, the LTE frame timing in the primary cell may be simultaneously used in the secondary cell.
Regulatory requirements, however, may not permit transmissions in the unlicensed spectrum without prior channel sensing. Since the unlicensed spectrum may need to be shared with other radios of similar or dissimilar wireless technologies, a so called Listen-Before-Talk (LBT) method may need to be applied. Today, the unlicensed 5 GHz spectrum is mainly used by equipment implementing the IEEE 802.11 Wireless Local Area Network (WLAN) standard. This standard is known under its marketing brand “Wi-Fi.”
In Europe, the LBT procedure is under the scope of EN 301.893 regulation. For LAA to operate in the 5 GHz spectrum, the LAA LBT procedure may conform to requirements and minimum behaviors set forth in EN 301.893. However, additional system designs and steps may be needed to ensure coexistence of Wi-Fi and LAA with EN 301.893 LBT procedures.
An example of an existing method is U.S. Pat. No. 8,774,209B2, “Apparatus and method for spectrum sharing using listen-before-talk with quiet periods,” where LBT is adopted by frame-based OFDM systems to determine whether the channel is free prior to transmission. A maximum transmission duration timer is used to limit the duration of a transmission burst, and is followed by a quiet period.
Long Term Evolution (LTE)
LTE uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink and Discrete Fourier Transform (DFT)-spread OFDM, also referred to as single-carrier Frequency Division Multiple-Access (FDMA), in the uplink. The basic LTE downlink physical resource may thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. The uplink subframe has the same subcarrier spacing as the downlink and the same number of Single-Carrier (SC)-FDMA symbols in the time domain as OFDM symbols in the downlink.
In the time domain, LTE downlink transmissions may be organized into radio frames of 10 millisecond (ms), each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms, as shown in FIG. 2. For normal cyclic prefix, one subframe consists of 14 OFDM symbols. The duration of each symbol may be approximately 71.4 μs.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot, 0.5 ms, in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction, 1.0 ms, may be known as a resource block pair. Resource blocks may be numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions may be dynamically scheduled, i.e., in each subframe the base may station transmit control information about which terminals data is transmitted to and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling may be typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe, and the number n=1, 2, 3 or 4 may be known as the Control Format Indicator (CFI). The downlink subframe may also contain common reference symbols, which are known to the receiver and used for coherent demodulation of, e.g., the control information. A downlink system with CFI=3 OFDM symbols as control is illustrated in FIG. 3, where the three OFDM symbols are indicated as control region. The control signaling is transmitted in the first OFDM symbol, as indicated.
From LTE Rel-11 onwards, above described resource assignments may also be scheduled on the enhanced Physical Downlink Control Channel (EPDCCH). For Rel-8 to Rel-10, only the Physical Downlink Control Channel (PDCCH) may be available.
The reference symbols shown in the above FIG. 3 may be the cell specific reference symbols (CRS) and may be used to support multiple functions, including fine time and frequency synchronization and channel estimation for certain transmission modes.
Physical Downlink Control Channel (PDCCH) and Enhanced PDCCH (EPDCCH)
The PDCCH/EPDCCH may be used to carry Downlink Control Information (DCI), such as scheduling decisions and power-control commands. More specifically, the DCI may include:
a) Downlink scheduling assignments, including PDSCH resource indication, transport format, hybrid-Automatic Repeat reQuest (ARQ) information, and control information related to spatial multiplexing, if applicable. A downlink scheduling assignment may also include a command for power control of the Physical Uplink Control Channel (PUCCH) used for transmission of hybrid-ARQ acknowledgements in response to downlink scheduling assignments.
b) Uplink scheduling grants, including Physical Uplink Shared Channel (PUSCH) resource indication, transport format, and hybrid-ARQ-related information. An uplink scheduling grant may also include a command for power control of the PUSCH.
c) Power-control commands for a set of terminals as a complement to the commands included in the scheduling assignments/grants.
One PDCCH/EPDCCH may carry one DCI message containing one of the groups of information listed above. As multiple terminals may be scheduled simultaneously, and each terminal may be scheduled on both downlink and uplink simultaneously, there may need to be a possibility to transmit multiple scheduling messages within each subframe. Each scheduling message may be transmitted on separate PDCCH/EPDCCH resources, and consequently, there may be typically multiple simultaneous PDCCH/EPDCCH transmissions within each subframe in each cell. Furthermore, to support different radio-channel conditions, link adaptation may be used, where the code rate of the PDCCH/EPDCCH may be selected by adapting the resource usage for the PDCCH/EPDCCH, to match the radio-channel conditions.
Carrier Aggregation
The LTE Rel-10 standard may support bandwidths larger than 20 MegaHertz (MHz). One requirement on LTE Rel-10 may be to assure backward compatibility with LTE Rel-8. This may also include spectrum compatibility. That may imply that an LTE Rel-10 carrier, wider than 20 MHz, may appear as a number of LTE carriers to an LTE Rel-8 terminal. Each such carrier may be referred to as a Component Carrier (CC). In particular, for early LTE Rel-10 deployments it may be expected that there may be a smaller number of LTE Rel-10-capable terminals, compared to many LTE legacy terminals. Therefore, it may be necessary to assure an efficient use of a wide carrier also for legacy terminals, i.e. that it may be possible to implement carriers where legacy terminals may be scheduled in all parts of the wideband LTE Rel-10 carrier. The straightforward way to obtain this may be by means of Carrier Aggregation (CA). CA may imply that an LTE Rel-10 terminal may receive multiple CC, where the CC have, or at least the possibility to have, the same structure as a Rel-8 carrier. CA is illustrated in FIG. 4. Note an aggregated bandwidth of 100 MHz is shown as an aggregation of five CC, each of 20 MHz, and therefore each individual CC may be handled by a terminal from an earlier release than LTE Rel-10. A CA-capable UE may be assigned a primary cell (PCell) which may be always activated, and one or more secondary cells (SCells) which may be activated or deactivated dynamically.
The number of aggregated CC as well as the bandwidth of the individual CC may be different for uplink and downlink. A symmetric configuration refers to the case where the number of CCs in downlink and uplink is the same whereas an asymmetric configuration refers to the case that the number of CCs is different. It is important to note that the number of CCs configured in a cell may be different from the number of CCs seen by a terminal: A terminal may for example support more downlink CCs than uplink CCs, even though the cell is configured with the same number of uplink and downlink CCs.
In addition, a feature of carrier aggregation is the ability to perform cross-carrier scheduling. This mechanism may allow a (E)PDCCH on one CC to schedule data transmissions on another CC by means of a 3-bit Carrier Indicator Field (CIF) inserted at the beginning of the (E)PDCCH messages. For data transmissions on a given CC, a UE may expect to receive scheduling messages on the (E)PDCCH on just one CC—either the same CC, or a different CC via cross-carrier scheduling; this mapping from (E)PDCCH to PDSCH may also be configured semi-statically.
Wireless Local Area Network
In typical deployments of WLAN, Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) may be used for medium access. This means that the channel may be sensed to perform a Clear Channel Assessment (CCA), and a transmission may be initiated only if the channel is declared as Idle. In case the channel is declared as Busy, the transmission may be deferred until the channel is deemed to be Idle. When the range of several Access Points (APs) using the same frequency may overlap, this means that all transmissions related to one AP may be deferred in case a transmission on the same frequency to or from another AP which is within range may be detected. Effectively, this means that if several APs are within range, they may have to share the channel in time, and the throughput for the individual APs may be severely degraded. A general illustration of the Listen-Before-Talk (LBT) mechanism is shown in FIG. 5.
After a Wi-Fi station (STA) A transmits a data frame to a station B, represented in the figure by the two wavy lines and the indication Busy Wireless Medium (WM), station B may transmit the ACK frame back to station A with a delay of 16 microseconds (μs), the so-called Short Inter-frame Spacing (SIFS). The SIFS duration may be understood as representing the nominal time, in μs, that the Wi-Fi Medium Access Control (MAC) and PHysical Layer (PHY) may require in order to receive the last symbol of a frame at the air interface, process the frame, and respond with the first symbol on the air interface of the earliest possible response frame. Such an ACK frame may be transmitted by station B without performing an LBT operation. To prevent another station interfering with such an ACK frame transmission, a station may defer for a duration of 34 μs, referred to as Distributed Coordination Function Inter-frame Spacing (DIFS), after the channel is observed to be occupied before assessing again whether the channel is occupied. This is represented in FIG. 5 as defer access.
Therefore, a station that wishes to transmit, may first perform a CCA by sensing the medium for a fixed duration DIFS. If the medium is idle, then the station may assume that it may take ownership of the medium and begin a frame exchange sequence. If the medium is busy, the station may wait for the medium to go idle, defer for DIFS, and wait for a further random backoff period.
To further prevent a station from occupying the channel continuously and thereby prevent other stations from accessing the channel, it may be required for a station wishing to transmit again after a transmission is completed to perform a random backoff. The random backoff is a procedure performed based on a so called Contention Window, wherein a random number of slots wherein the channel is to be found idle before transmission may take place is drawn from the range that may be specified by the Contention Window. This number may be counted down as long as the medium is found to be idle, and the counter may be frozen when the medium is found to be busy. When the count goes down to zero, transmission, e.g., of data, as shown in the Figure, may start. The Contention Window may be increased if previous transmissions are not received successfully by the intended recipient, or reset to a nominal value when previous transmissions are received successfully.
The Point Coordination Function Inter-frame Spacing (PIFS) may be used to gain priority access to the medium, and is shorter than the DIFS duration. Among other cases, it may be used by STAs operating under PCF, to transmit Beacon Frames with priority. At the nominal beginning of each Contention-Free Period (CFP), where access to the medium may be coordinated by a Point Coordinator (PC), the PC may sense the medium. When the medium is determined to be idle for one PIFS period, generally 25 μs, the PC may transmit a Beacon frame containing the Contention-Free (CF) Parameter Set element and a delivery traffic indication message element. The CF parameter set may carry parameters that may be needed to support PCF operation. A delivery traffic indication map may be understood as a traffic indication map which may inform the STAs about the presence of buffered multicast/broadcast data on the AP.
Load-based Clear Channel Assessment in Europe Regulation EN 301.893
For a device not utilizing the Wi-Fi protocol, EN 301.893, v. 1.7.1 provides the following requirements and minimum behavior for the load-based clear channel assessment. An example to illustrate the EN 301.893 is provided in FIG. 6.
1) Before a transmission or a burst of transmissions on an Operating Channel, the equipment may perform a Clear Channel Assessment (CCA) check using “energy detect”, as represented in the Figure by a circled “1”. The equipment may observe the Operating Channel(s) for the duration of the CCA observation time, which may be not less than 20 μs. The CCA observation time used by the equipment may be declared by the manufacturer. The Operating Channel may be considered occupied if the energy level in the channel exceeds the threshold corresponding to the power level given in point 5 below. If the equipment finds the channel to be clear”, as represented in the Figure by a circled “1”, it may transmit immediately,”, as represented in the Figure by a circled “2”, see point 3 below.
2) If the equipment finds an Operating Channel occupied, it may not transmit in that channel. The equipment may perform an Extended CCA check”, as represented in the Figure by a circled “3”, in which the Operating Channel is observed for the duration of a random factor N multiplied by the CCA observation time. N defines the number of clear idle slots resulting in a total Idle Period that may need to be observed before initiation of the transmission. The value of N may be randomly selected in the range 1 . . . q every time an Extended CCA may be required, and the value stored in a counter. The value of q is selected by the manufacturer in the range 4 . . . 32. This selected value may be declared by the manufacturer, see clause 5.3.1 q. The counter is decremented every time a CCA slot is considered to be “unoccupied”. When the counter reaches zero, the equipment may transmit”, as represented in the Figure by a circled “2”, on the right side.
NOTE 2: The equipment is allowed to continue Short Control Signalling Transmissions on this channel providing it complies with the requirements in clause 4.9.2.3.
NOTE 3: For equipment having simultaneous transmissions on multiple, adjacent or non-adjacent, operating channels, the equipment is allowed to continue transmissions on other Operating Channels, providing the CCA check did not detect any signals on those channels.
3) The total time that an equipment makes use of an Operating Channel is the Maximum Channel Occupancy Time which may be less than (13/32)×q ms, with q as defined in point 2 above, after which the device may perform the Extended CCA described in point 2 above.
4) The equipment, upon correct reception of a packet which was intended for this equipment, can skip CCA and immediately, see note 4, proceed with the transmission of management and control frames (Ctrl), e.g. ACK and Block ACK frames”, as represented in the Figure by a circled “4”. A consecutive sequence of transmissions by the equipment, without it performing a new CCA, may not exceed the Maximum Channel Occupancy Time as defined in point 3 above.
NOTE 4: For the purpose of multi-cast, the ACK transmissions, associated with the same data packet, of the individual devices are allowed to take place in a sequence
5) The energy detection threshold for the CCA may be proportional to the maximum transmit power (PH) of the transmitter: for a 23 decibel-milliwatts (dBm) Effective Isotropic Radiated Power (e.i.r.p.) transmitter, the CCA threshold level (TL) may be equal or lower than −73 dBm/MHz at the input to the receiver, assuming a 0 decibel isotropic (dBi) receive antenna. For other transmit power levels, the CCA Threshold Level (TL) may be calculated using the formula: TL=−73 dBm/MHz+23−PH, assuming a 0 dBi receive antenna and PH specified in dBm e.i.r.p.
Licensed Assisted Access (LAA) to Unlicensed Spectrum Using LTE
Up to now, the spectrum used by LTE is dedicated to LTE. This has the advantage that an LTE system may not need to care about coexistence with other non-3GPP radio access technologies in the same spectrum and spectrum efficiency may be maximized. However, the spectrum allocated to LTE is limited, which cannot meet the ever increasing demand for larger throughput from applications/services. Therefore, a new study item has been initiated in 3GPP on extending LTE to exploit unlicensed spectrum in addition to licensed spectrum.
With Licensed-Assisted Access to unlicensed spectrum, as shown in FIG. 7, a UE may be connected to a PCell in the licensed band and one or more SCells in the unlicensed band. In this application a secondary cell in unlicensed spectrum may be denoted as LAA secondary cell (LAA SCell). The LAA SCell may operate in DL-only mode or operate with both UL and DL traffic. Furthermore, in future scenarios, the LTE nodes may operate in standalone mode in license-exempt channels without assistance from a licensed cell. Unlicensed spectrum may, by definition, be simultaneously used by multiple different technologies. Therefore, LAA as described above may need to consider coexistence with other systems such as IEEE 802.11 (Wi-Fi).
To coexist fairly with the Wi-Fi system, transmission on the SCell may conform to LBT protocols in order to avoid collisions and causing severe interference to on-going transmissions. This may include both performing LBT before commencing transmissions, and limiting the maximum duration of a single transmission burst. The maximum transmission burst duration may be specified by country and region-specific regulations, for e.g., 4 ms in Japan and 13 ms according to EN 301.893. An example in the context of LAA is shown in FIG. 8, with different examples for the duration of a transmission burst on the LAA SCell constrained by a maximum allowed transmission duration of 4 ms. FIG. 8 is a schematic diagram illustrating LAA to unlicensed spectrum using LTE carrier aggregation and listen-before-talk to ensure good coexistence with other unlicensed band technologies. In FIG. 8, the transmitted bursts are represented with black rectangles. Each rectangle represents a subframe. Note that before every transmitted burst in the SCell, a listening period is performed, as indicated by the striped areas. Bursts of 4 ms, 3 ms and 8 ms are represented in the Figures, as examples. Because in the example of FIG. 7, the maximum allowed transmission duration of 4 ms, the 8 ms burst is interrupted by a listening period after the first 4 ms of the burst.
Existing methods for LAA LTE to support LBT in unlicensed spectrum may comprise inappropriate delays of transmission, as well as interference problems, that result in poor performance of a wireless communications network.